A review of the scientific literature clarifies several chronobiological features of dreaming. The literature supports the conclusions that dreaming ‘intensity’ and, to a lesser extent dream-like quality, is modulated by (1) a sinusoidal, 90-min ultradian oscillation, (2) a ‘switch-like’ circadian oscillation, (3) a 12-h circasemidian rhythm, and (4) a 28-day circatrigintan rhythm (for women). Further, access to dream memory sources appears to be modulated by (5) a 7-day circaseptan rhythm. Further study of these rhythmic influences on dreaming may help to explain diverse and often contradictory findings in the dream research literature, to clarify relationships between dreaming and waking cognitive processes, to explain relationships between disturbed phase relationships and dream disturbances and to shed new light on the problems of dreaming's functions and biological markers. Further chronobiological studies of dreaming will likely enable the development of theoretical models that explain how interactions between and within major levels of oscillation determine the variable characteristics of dreaming.
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